1. Field of the Invention
The present invention relates to an optical disk drive for writing data to, and reading data from, a plurality of types of optical disks having different recording densities, an optical pickup device usable for such an optical disk drive, and an objective lens driving device usable for such an optical pickup device; and more particularly relates to an objective lens driving device, usable for an optical pickup device, having a plurality of objective lens mounted thereon.
2. Description of the Related Art
As disk-type recording mediums (optical disks) usable for data write and/or data read utilizing light, CDs (Compact Discs) and DVDs (Digital Versatile Discs) are currently in wide use. In addition to these disks, BDs (Blu-ray Discs) and other optical disks having a high recording density have been recently proposed and developed. The wavelength of used for such optical disks is 780 nm or the vicinity thereof for CDs, 660 nm or the vicinity thereof for DVDs, and 405 nm or the vicinity thereof for BDs or the like.
It is desired that one optical pickup device should be usable to perform data recording to, and data reproduction from, a plurality of types of optical disks used with different wavelengths of laser beam. In one example of an optical pickup device proposed to realize this, different objective lenses respectively prepared for CDs/DVDS and BDs are mounted on one objective lens driving device.
FIG. 10A shows an exemplary objective lens driving device, in a conventional optical pickup device, having two types of objective lenses mounted thereon. Such an objective lens driving device is described in, for example, Japanese Laid-Open Patent Publication No. 2003-281758.
A lens holder 300 includes a first objective lens setting opening 301 and a second objective lens setting opening 302, which are respectively formed to allow a first objective lens and a second objective lens (not shown) to be mounted therein.
FIG. 10B shows a first through-hole 303 and a second through-hole 304 which pass through the lens holder 300. The first objective lens setting opening 301 and the second objective lens setting opening 302 are respectively open ends of the first through-hole 303 and the second through-hole 304. It is not necessary that the first objective lens setting opening 301 and the second objective lens setting opening 302 have the same diameter or that the first through-hole 303 and the second through-hole 304 have the same diameter.
The first through-hole 303 and the second through-hole 304 are optical paths for laser beam incident from a light source (not shown) provided below the first through-hole 303 and the second through-hole 304 in a focusing direction. The laser beam which has passed through the first or second objective lens is focused at an optical disk (not shown) provided above the first through-hole 303 or the second through-hole 304 in the focusing direction, and the light reflected by the optical disk returns in the corresponding path in the opposite direction back to a detector (not shown) provided on the light source side.
The lens holder 300 also includes a magnetic circuit elastically supported by a spring (not shown) for driving the first or second objective lens independently in the focusing direction perpendicular to a recording surface of the optical disk and in a tracking direction matching a radial direction of the optical disk.
The recent demand for a higher recoding/reproduction speed requires an increased driving force, which in turn requires a movable section including the lens holder 300 to be more lightweight, and accordingly, to have a higher sensitivity. The movable section is also required to have a higher rigidity and thus to have a higher resonance frequency in a high frequency range, so that the movable section is compatible with a control system having an increased gain. Namely, the objective lens driving device is desired to have opposing characteristics of a less weight and a higher rigidity.
In an optical pickup device having two types of objective lenses mounted thereon, the two lenses often have different weights. The material for the lenses is preferably a plastic material for reducing the cost and weight, but glass is occasionally chosen as the material of the objective lens used for laser beam having a wavelength of 405 nm from the viewpoint of the resistance against light. Assuming, for example, the objective lens used for DVDs is formed of a plastic material, the glass objective lens may be three time or more heavier than the plastic objective lens due to the difference in the specific gravity between glass and plastics.
When such lenses having different weights are mounted on the lens holder 300, the weight balance is destroyed and the gravity center is shifted from the center of the movable section. If the entire lens holder 300 is sufficiently large to optimize the positions of the individual lenses, it is possible to set the gravity center at the center of the lens holder 300, which matches the center of the driving force generated by the magnetic circuit.
However, it is preferred that the lens holder 300 is as small as possible in order to fulfill the recent demands for a less weight and a higher rigidity required for an operation in more highly multiplied speed modes. It is also desired that the two lenses are located as close as possible to each other, which significantly restricts the designing freedom. Therefore, it is difficult to optimize the weight balance by lens arrangement.
Where the weight balance is destroyed and thus the gravity center is shifted from the center of the movable section (driving force center), the following problems occur. When a driving force generated by the magnetic circuit is acted on the movable section, a rotation moment is generated due to the center of the driving force being shifted from the gravity center. This causes unnecessary resonance and thus destabilizes the control on the objective lens driving device.
For example, in the example shown in FIG. 10A, the objective lenses are located side by side in the tracking direction. Therefore, the weight balance in the tracking direction is destroyed. As a result, when the movable section is entirely driven in the focusing direction, rolling resonance is generated by which the movable section is rotated with a tangential direction being used as the rotation axis. In a different example where the objective lenses are located side by side in the tangential direction, when the movable section is entirely driven in the focusing direction, pitching resonance is generated by which the movable section is rotated with the tracking direction being used as the rotation axis.